The term yantra, which literally
means an instrument for holding or restraining, may be used to denote a
variety of linear diagrams which play a significant role in the meditative
practices of Tantric Hinduism.

Yantras may be simple designs such as the cross,
triangle, square, circle or lotus
pattern, symbolizing basic concepts, or may be more complex combinations
of such elements in figures representing in abstract form the particular
creative forces in the cosmos which are called divinities. they are closely
related to the mandalas
used by both Hindu and Buddhist Tantrism, in which geometric design is
supplemented by elaborate symbolic images of the deities which by their
various forms and attributes indicate different aspects of the hidden order
of reality.
As Mircea Eliade says, the yantra is 'the linear paradigm of the mandala',
expressing the same principles in geometric form. Like mandalas, yantras
are used in the context of meditation and worship as visual-aids to concentration
of the mind leading to realization of abstract principle which is the inner
meaning of the visible representation.

The best known and geometrically
the most complex yantra is the Sri-yantra, also known as the Sri-yantra,
employed by the Sakta school of Tantrism which visualizes the divine primarily
in female form. The structure of this yantra is enigmatically described
in the Saundarya-lahari (The Wave of Beauty), a lengthy poem praising the
great goddess whose dwelling place the Sri-yantra is said to be:

By reason of the four Srikanthas (srikantha is
an epithet of Siva)
and the five damsels of Siva (which have the nature of Sakti), which are
penetrated by Sambhu (i.e. bindu- the dot in the centre) and constitute
the nine fundamental natures, the 43 (or 44) angles of your dwelling place
are evolved,
along with the 8-petalled
and 16-petalled lotuses, the circles and the three lines. (stanza 11)

The diagram may be more accurately
described as a bilaterally symmetrical figure composed of nine interwoven
isosceles triangles, usually depicted with five triangles pointing downwards
and four pointing upwards. The former are said to correspond to the yoni
representing the dynamic female principle of energy (Sakti), while the
latter correspond to the linga representing the static male principle of
wisdom (Siva). (The Buddhist Tantrics, incidentally, regard the male principle
as dynamic and the female as static.) The central dot called bindu represents
the original unity of the male and female principles prior to creation
and the paradoxical point female principles prior to creation and the paradoxical
point from which the manifestation of the cosmos emerges. The interpenetration
of the nine basic triangles gives rise to a number of subsidiary triangles
(43 including the central triangle enclosing the bindu) which form the
abodes of the deities, representing the particularization of the original
creative forces into more concrete manifestations. Sometimes the names
of deities and Sanskrit
syllables are written into these triangles, or images of the deities are
placed in them.

In most versions of the yantra this central design
is enclosed by two circular lotus-patterns with eight
and sixteen petals, a girdle of three concentric circles, and finally a
square arrangement of straight line ('the three lines') with four openings
or 'doors' at the cardinal points called 'World House' (bhugra). This square
outline, which is common also to mandalas, symbolizes the royal palace
in which the deities reside - an area of sacred space protected from the
disintegrating forces of chaos.
In general, the Sri-yantra is a 'cosmogram' - a graphic representation
of the universal processes of emanation and reabsorption reduced to their
essential outline. As Eliade puts it, the yantra: 'An expression in terms
of linear symbolism of the cosmic manifestations, beginning with the primordial
unity.'

There is apparently a Sri Yantra
land art formation in southeast Oregon - it consists of 13 miles of lines